As the newly created industry of mobile computing begins to mature, the need for wireless data communications has become increasingly apparent. Accurate, high-speed digital transmission is beginning to meet this need, but not without encountering difficult problems in the areas of accuracy, and in particular, transient errors.
In systems used to transmit digital information, the modulating signal can often be viewed as varying amplitude levels about a reference level (the DC part of the signal). The amplitude of the various levels in some manner encodes the desired information, and in such systems it is therefore necessary at the transmitter to be able to precisely control the amplitude of these levels.
Furthermore, in packet data systems designed to transmit digital information in bursts of a signal, possibly on a time-multiplexed shared channel, it is necessary to maintain these levels accurately throughout the duration of the signal burst. For any number of practical reasons, energy storage elements within the transmitting circuitry will cause transient errors to be imposed upon the desired signal levels, ultimately leading to errors in the received information. In such systems it is necessary to generate signals with controlled amplitude levels in a manner which minimizes this startup transient at the beginning of a packet burst.
To accomplish this, it is desired that the initial DC value of the signal generator, at the instant before the start of the signal burst, be equal to the average DC value of the signal during the data burst. This requires that the controlled amplitude levels of the signal be adjusted symmetrically about the DC reference, and that the generator is held near the DC reference level during the intervals when not actively generating a data burst.
Any circuitry used to accomplish the above two goals of accurate level generation and minimum startup transients must furthermore not corrupt the desired signal in any way; e.g. limit the frequency response or add any amplitude or time dependent distortions. It is also desirable that the adjustment of the accurate levels be accomplished easily during manufacture, or adaptively during actual field operation.
Towards these ends, practitioners have employed various circuits using manually adjustable potentiometers, electronic attenuators, digital to analog converters, etc., generally without totally satisfactory performance, particularly with the very high data rates and fast packet times of modern digital communications systems. Methods which set the signal levels by using attenuators within the signal path will affect the DC level of the signal, causing undesired startup transients. Also attenuation within the signal path can cause undesired frequency response roll-off or other distortions. Calculating the variable signal levels by performing mathematical operations within a digital signal processor, and generating the actual signal levels using a digital to analog converter, is prohibitively expensive, particularly for very high speed data waveforms.
Accordingly, a need exists for an apparatus that accurately controlling data signal levels while minimizing undesired transient effects.